Contract needs dates for conclusion, start and end

[JuergenGrupp]

The class contract does not have a property to set the date, neither for when the contract was concluded ( or signed) nor for when the matter of the contract becomes effective or expires.
Proposal: add three data properties:

• hasDateOfConclusion
• hasDateToBecomeEffective
• hasDateToExpire
  All properties require data type time:instant

[JuergenGrupp]

EBU Editorial Committee: proposal for better terms

• hasDateCreated (to be used as in Asset)
• hasDateValidFrom ( not: hasValidityDate or effective or or hasDateOfValidity or hasDateEffective)
• hasDateOfExpiry (not: hasExpiry or expires)

Probably: more dates will be required, so a good naming convention is a good thing to have ...